Enhancement of electric fields around metal particles is a topic of current scientific and technological interest. For example, surface enhanced Raman spectroscopy (“SERS”) is a well-known spectroscopy technique that utilizes an enhanced electric field near a specially prepared, roughened metal surface or metal particles to increase a Raman signal from an analyte. In SERS, the analyte is adsorbed onto, or placed adjacent to, an activated metal surface or structure. Irradiation of the analyte and the metal surface or particles with light of a selected frequency excites surface plasmons in the metal surface or particles. The surface plasmon frequency is relatively independent of surface or particle geometry and is predominately a function of the composition of the metal.
During SERS, the analyte experiences the intense, localized electric field of the surface plasmons, and Raman photons characteristic of the analyte are scattered from the analyte. The enhanced electric field is considered one significant factor for the relatively increased Raman signal compared to when Raman spectroscopy is practiced without the metal surface or particles. For example, the enhanced electric field from the metal surface may enhance the Raman scattering intensity by factors of between 103 and 106.
Recently, Raman spectroscopy has been performed employing randomly oriented metal nanoparticles, such as nanometer scale needles, islands, and wires, as opposed to a simple roughened metal surface, for enhancing electric fields. The intensity of the Raman scattered photons from a molecule adsorbed on such a metal surface may be increased by factors as high as 1016. At this level of sensitivity, Raman spectroscopy has been used to detect single molecules and is commonly referred to as nano-enhanced Raman spectroscopy (“NERS”).
As can be appreciated from the discussion above about SERS and NERS, enhancement of electric fields around metal particles can be of significant utility. In addition to SERS and NERS, enhancement of electric fields can be used in other applications, such as sensors, Raman imaging systems, nanoantennas, and many other applications. Regardless of the particular application, electric field enhancement using metal particles or surfaces has several limitations. The frequency at which light can be coupled to either localized or surface plasmons is relatively independent of the surface or particle geometry and is predominately a function of the composition of the metal. Thus, altering the size or geometry of the metal surface or metal particles has only a minor effect on the frequency at which light can be coupled to the surface plasmons. Accordingly, the frequency at which light can be coupled to surface plasmons is essentially fixed by the composition of the metal surface or particles, which limits their usefulness in many applications.
In addition to lack of scalability, many types of metal nanoparticles are known to be toxic. Metal nanoparticle toxicity can make safe-manufacturing of electric-field-enhancement structures difficult, and may limit application of electric-field-enhancement structures including metal nanoparticles in certain biomedical applications. Furthermore, fabrication of electric-field-enhancement structures with metal particles typically relies on a self-assembled distribution of the metal nanoparticles. Thus, it can be difficult to precisely space or align metal nanoparticles.
Therefore, researchers and developers of electric-field-enhancement structures can appreciate a need for a scalable and less-toxic electric-field-enhancement structure for use in a wide variety of applications, such as sensors, Raman spectroscopy systems, and many other applications.